1. Field of the Invention
The present invention relates to the field of sound absorption devices.
2. Prior Art
Resonators were first used by the ancient Greeks to reduce echoes in their large open air theaters. By the thirteenth century, resonators were used in churches in Sweden and Denmark, centuries before Helmholtz developed the first mathematical model of their behavior. Helmholtz resonators, as they are now known today, are currently being used as sound absorbing devices in a variety of commercial applications, including aircraft engines, auditoriums, concert halls and in compressor inlet and exhaust mufflers.
The classical Helmholtz resonator comprises an air cavity coupled to the outside space through some form of opening such as an orifice, slot, tube, or the like. The compressibility of the air within the cavity acts as a spring, with the air flowing in the opening acting as a mass so that the system will be tuned as a spring-mass system to an acoustic frequency dependent upon these two parameters.
When Helmholtz resonators are driven with acoustic energy at the resonant frequency, the resonators will absorb a maximum amount of the incoming acoustic energy. However, because they are tuned systems, the absorption decreases rapidly as the frequency of the incoming acoustic energy varies substantially from the resonant frequency. Thus the principle limitation of these devices is that they absorb sound energy efficiently only within a narrow frequency range centered at their tuned frequency. Therefore they can control only one acoustic mode excited at a single frequency. While this is suitable for some applications, such as rotating machinery which operates at a substantially constant angular velocity, it is far from ideal for equipment such as aircraft engines whose angular velocity may vary substantially between waiting for take off instructions, take off conditions, cruise and approach conditions. In that regard, in general the noise emitted by jet engines includes not only the reasonably white noise in the exhaust, but further includes components which are directly proportional to engine speed, and many strong components which are harmonics of engine speed, such as turbine blade passing frequencies, etc.
One approach to this problem is disclosed in U. S. Pat. No. 3,972,383. That patent discloses a system for varying the acoustic resistance of an acoustical lining disposed in a duct of an air propulsar. The system comprises a nonlinear sound suppression liner having a porous facing sheet overlying a plurality of cells, and means for impinging a predetermined oscillatory air pressure signal of 100-160 db at an inaudible frequency on the facing sheet to vary the acoustic resistance of the facing sheet to make it optimum for a selected sound level and air flow condition in the duct. It would appear that the result achieved is similar to that which would result from being able to mechanically adjust the openings of the liner, namely to change the frequency for best absorption of the liner as may be required for the variations occurring between take off, cruise and approach. Such an arrangement, however, would not broaden the frequency band for best absorption, thereby allowing one resonator to absorb a plurality of frequencies over a reasonable band at one time, a primary objective of the present invention.
In addition to the forgoing, various other types of active noise control techniques, generally in the form of noise cancellation techniques, are also well known. In accordance with these techniques, a microphone is used to sense sound, normally a single tone being emitted by the noise source, with the microphone signal being amplified and phase shifted an appropriate amount to power a driver to generate an equal and opposite sound component of appropriate phase to cancel the original sound. In certain applications and under appropriate conditions, substantial sound cancellation may be achieved in this manner. However, such a technique has certain limitations which in various applications are either undesirable or in some instances preclude the use thereof. By way of example, the power requirements both in terms of power itself and the required support equipment are very substantial, as the acoustic energy which must be generated must equal that to be cancelled, which may be quite high for large equipment such as turbines and the like. Further, the acoustic driver or drivers essentially form part of the wall of a duct or other chamber associated with the noise source, and accordingly the technique is not very compatible with circular ducts or particularly ducts having compound curvatures and the like. Also, in many applications the environment is too hostile for an acoustic driver to form a portion of a duct wall therein, such as by way of example, jet engine exhaust, rocket engine exhaust and the like. The present invention, as shall subsequently be seen, is not subject to the same limitations, as it requires much less power than the foregoing techniques and can be made compatible with, and therefore is applicable to, engine exhaust and the suppression of noise therein.